Fertilizers have the highest percentage of nutrients, so they're going to work faster than other methods. However, there are other practices you can follow to keep adding nitrogen back to your yard and garden; you'll likely avoid having a nutrient deficiency in the first place.
Grow a cover crop of peanuts or soy. Both peanuts and soy are high in nitrogen and they replenish the supply in the soil. After you mow down your garden in the late fall, plant it with peanuts or soy. They might not make it to maturity, depending on the weather in your area, but that's OK. Let them grow as long as they survive and leave them on the soil until the ground thaws, then till them under as an addition to the soil.
Add organic compost to the soil. Organic matter is rich in nitrogen, so working in a well-cured supply of compost adds nitrogen back into the earth.
Return dried grass clippings into the soil. Grass clippings are a rich source of nitrogen. Use a mulching mower, which cuts the clippings down very fine and helps them to spread out, dry, and break down into the soil without becoming a soggy mass.
Grow and mow down comfrey. Comfrey is an herb that is very high in nitrogen; it works great when thrown into a compost pile to help all the elements decompose. If you have an unused corner of your yard or garden, plant some comfrey, then simply mow or cut it down and work it back into the soil. Beware, however: comfrey can become invasive. Keep it in check or grow it in a container.
Mulch with peanut shells and dried grass clippings. Peanut shells, like grass clippings, are high in nitrogen content. Use both or either as an effective mulch; they'll help reduce weed growth and enrich the soil.
Pour brewery waste and/or coffee on the area. If you have any distilleries in the area, you can probably get your hands on some of the brewery waste which is simply disposed of. It's high in nitrogen. Spread it around your plants. Pour your old coffee out around shrubs or in your watering can for the house plants.